|Publication number||US20060069522 A1|
|Application number||US 10/970,580|
|Publication date||Mar 30, 2006|
|Filing date||Oct 20, 2004|
|Priority date||Sep 27, 2004|
|Also published as||CA2484951A1, US7212930|
|Publication number||10970580, 970580, US 2006/0069522 A1, US 2006/069522 A1, US 20060069522 A1, US 20060069522A1, US 2006069522 A1, US 2006069522A1, US-A1-20060069522, US-A1-2006069522, US2006/0069522A1, US2006/069522A1, US20060069522 A1, US20060069522A1, US2006069522 A1, US2006069522A1|
|Original Assignee||Veris Industries, Llc|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (2), Referenced by (5), Classifications (5), Legal Events (3)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The present invention relates to a method and apparatus for determining the relative phase of two waveforms and, more particularly, to an electricity metering method and apparatus for determining the phase of distorted voltage and current waveforms.
Electric power is typically generated at a remote, central generating facility and transported to the consumer over a distribution system. To reduce power transportation losses, a step-up, subtransmission transformer is used to increase the voltage and reduce the current for transmission over a transmission line. The actual transmission line voltage usually depends on the distance between the subtransmission transformers and the consumers of the electricity but is commonly in the range of 2-35 kilo-volts (“kV”). Distribution substation transformers and distribution transformers of an electric utility's secondary power distribution system reduce the voltage from the transmission line level to a distribution voltage for delivery and use by industrial, commercial, and residential customers. In the United States, for example, electric power is typically delivered to the consumer as a alternating current (AC) voltage ranging from 120 volts (“V”) to 660 V, depending upon the use. As generated, the fundamental AC voltage and current approximate 60 Hertz (“Hz”) sine waves over time.
The consumption of power by individual consumers and the performance of the distribution system are monitored by power meters. Power meters are used to monitor a number of electrical parameters related to power distribution and use, including the active power, the time rate of transferring or transforming energy, and the apparent power, the product of the root mean square (RMS) voltage and current. In addition, the reactive power, the product of the RMS voltage and the quadrature component of the current, and the power factor or quality factor, the ratio of active power to apparent power, are commonly monitored.
Power meters may also be used to measure the phase angle between the current and voltage waveforms. The phase angle expresses the temporal relationship of a cycle distinguishing feature of a first waveform, commonly an amplitude peak or a zero crossing, to the position of a corresponding feature in another waveform. As generated, the voltage and current waveforms are in phase and simultaneously reach zero, maximum, and minimum amplitudes. If the load is purely resistive, the voltage and current remain in phase. However, if the load is capacitive, the phase distinguishing feature of the current waveform will precede that of the voltage waveform and the phase angle is designated as leading. On the other hand, an inductive load produces a lagging phase angle with the phase distinguishing feature of the voltage waveform preceding that of the current waveform. The reactive power consumed by capacitive and inductive loads causes power losses in the transmission system and reduces the overall efficiency of the power distribution system. Phase angle measurement permits billing the consumer for reactive power losses and analysis of the nature of the load so that power conditioning equipment can be added to the system to reduce the reactive power losses.
Inductive, capacitive, and resistive loads have impedances that are independent of voltage and at any single frequency the impedances of these loads are linear. While an inductive or capacitive load will cause the relative phase angle of the voltage and current to change, the sinusoidal voltage and current waveforms are not distorted when an AC voltage is applied to an inductive, capacitive, or resistive load. Sinusoidal waveforms have definite zero crossings and amplitude peaks and, typically, either a zero crossing or an amplitude peak is selected as the distinguishing feature to temporally mark the cycles of a waveform when determining the phase angle or phase.
What is desired, therefore, is a method and apparatus for accurate and consistent determination of the phase angle relative to a harmonically distorted waveform.
Referring in detail to the drawings where similar parts of the invention are identified by like reference numerals, and, more particularly, to
The sinusoidal AC voltage and current waveforms are defined, respectively, by the equations:
v=V m cos(ωt+θv) (1)
i=I m cos(ωt+θ i) (2)
The outputs from the exemplary current transducer 96 and the exemplary voltage transducer 94 are input to a data processing system 104 through an analog-to-digital converter (ADC) 106. The data processing system 104 comprises a data processing device, typically, a microprocessor, programmable logic device (PLC), or digital signal processor, program instructions, and related data storage and peripheral devices. The data processing system 104 includes a memory 108 that is adapted to store instructions and data used by the data processing device in calculating the current, voltage, power, and other related electrical parameters (e.g., number of turns in the secondary winding, etc.). The data processing system 104 is also connected to a display system 110 permitting display of the metered quantities. A user interface may be incorporated in the display section permitting manual selection of different metered quantities for display. Other peripheral devices, components, and features may be provided including, for example, a communications interface 112 that enables remote reporting of the meter results or transmission of instrument data to a remote data processing system.
Digital, electronic power metering equipment typically calculates the power and other electrical parameters from a plurality instantaneous voltage and current amplitudes obtained by independently sampling the outputs of the voltage and current transducers. The outputs of the voltage and current transducers vary, respectively, with the variation of the voltage and current at the meter's terminals. Typically, sampling is controlled by the meter's data processing system so that it is performed periodically and at known times in relation to the sinusoidal fundamental waveform.
The relative phase or phase angle of a pair of periodic waves is determined by resolving the temporal difference in the occurrence of corresponding cyclic distinguishing features of the cycles of the two waveforms. As generated, the AC voltage and current waveforms are “in phase,” sine waves that simultaneously attain zero, maximum, and minimum amplitudes. If the load is purely resistive, the voltage and current remain in phase (θ=0). However, inductance or capacitance causes a temporal shifting of the sinusoidal current and voltage waveforms so that the peak and zero amplitudes of the waveforms no longer occur simultaneously (θ≠0). While inductance and capacitance cause a phase shift, a change in the phase angle, the impedance of resistive, capacitive, and inductive loads is linear and the sinusoidal current and voltage waveforms are not distorted by the load. Since a sine wave exhibits discrete zero crossings and amplitude peaks, a zero crossing or amplitude peak of one of the waveforms, a reference waveform, is commonly used to temporally distinguish the cycles of the reference waveform so that the interval and, consequently, the phase angle to a corresponding zero crossing or amplitude peak of the other waveform can be measured.
Increasingly, however, the loads imposed on electrical distribution systems comprise power electronic loads, including AC and DC variable speed drives, power rectifiers and inverters, arc furnaces, and discharge lighting, such as fluorescent lighting. Typically, power electronic loads include a front end section comprising a rectification bridge and a parallel input filter capacitor. The front end section acts as a peak detector because current flows to charge the capacitor only during a portion of each half cycle of the sine wave when the instantaneous AC voltage exceeds the voltage on the capacitor. Between these current peaks, the load draws energy stored in the capacitor.
The specific harmonics produced by a power electronic load are determined by the number of rectifiers (pulse number) used in the rectification circuit and can determined by the equation:
The present inventor concluded that extensive filtering would not be required and the data processing resources of a power meter could be substantially reduced if, for phase measurement, the cycles of a waveform were temporally distinguished at a trigger amplitude that is greater than zero but less than a peak amplitude of the waveform. The amplitudes of the harmonics 22, 24, 26, 28 are substantially less than the amplitude of the fundamental waveform and are not arithmetically summed but combined by root mean square (RMS) summation, as follows, to determine the instantaneous amplitude of the harmonically distorted resultant waveform:
V rms=(V 1 2 +V 2 2 +V 3 2 + . . . +V n 2)1/2 (6)
The relative phase or phase angle of two periodic waveforms of known frequency is proportional to the time interval elapsing between the occurrences of amplitudes that distinguish the repetition of cycles of the waveforms. Referring to
On the other hand, occurrences of an amplitude equaling a trigger amplitude are used to distinguish cycles of the harmonically distorted reference waveform, the voltage 30. Even though the waveform is harmonically distorted, the cycles of the periodic wave are substantially symmetrical about the expected maximum amplitude of the fundamental frequency 38 which occurs at substantially one-half of the interval between the occurrences of the trigger amplitudes 36 on the increasing and decreasing slopes of the cycle. In addition, the harmonically distorted waveform is symmetrical about zero amplitude and the cycles can be temporally distinguished by occurrences of amplitudes having absolute values equal to the absolute value (either positive or negative amplitudes) of the trigger amplitude. To avoid false triggering resulting from the localized amplitude dithering, particularly in the vicinity of the expected zero crossing and amplitude peak of the fundamental frequency component of the harmonically distorted reference waveform, the trigger amplitude is set at a level greater than 10% but less than 90% of the maximum amplitude of the fundamental frequency component of the waveform. Typically, an amplitude approximately intermediate between zero and the maximum amplitude of the fundamental frequency, for example 50V, is selected as a trigger amplitude because the resistance to false triggering is greatest and the temporal effects of harmonic distortion are minimized at one-half of the maximum amplitude.
In the power meter 80, occurrences of a voltage equal to the trigger voltage 36 are detected by a trigger amplitude detector 114. The trigger amplitude detector 114 initiates operation of a first timer 116 when the absolute value of the instantaneous voltage 30 equals or exceeds the absolute value of the trigger voltage 36 on one edge of the cycle and terminates operation of the first timer when the absolute value of the voltage again drops below the absolute value of the trigger voltage on the opposite edge of the cycle. Referring to
If the peak current has been reached 154, the operating status of the first timer is also checked 160. If the first timer has not been started, timing is initiated with second timer 162 and a flag is set 164 indicating that the peak amplitude of the current was attained before the voltage 30 had reached the trigger amplitude 36.
If the current has attained its maximum amplitude 154 at the present sample and the first timer has been started 160, the data processing system checks the status of the flag 166. If the flag has been set 166, indicating that operation of the second timer was initiated before operation of the first timer was initiated; the data processing system waits 168 until the trigger amplitude detector 114 terminates operation of the first timer 170 and then terminates operation of the second timer 172. The data processing system 104 then calculates the phase angle from the first and second time intervals 174. Since the second timer was initiated before the voltage reached the trigger voltage 36, the phase is leading and the phase angle equals:
On the other hand, if, when sampled 152, the current has attained its maximum amplitude 154 and the first timer has been started 160, but the flag 166 indicates that the operation of the second timer was initiated when the voltage reached the trigger voltage 36 and operation of the first timer was initiated; the second timer is stopped 176. The data processing system waits 180 until the trigger amplitude detector 114 stops the first timer 178 and then calculates the phase 182 from the first and second time intervals. In this case, the phase angle equals:
If the second time interval is less than one-half the first time interval, the calculated phase will be positive, indicating a leading phase produced by a capacitive load. If the second timer interval is greater than one-half of the first timer interval, the phase angle will be negative indicating an inductive load.
While the first 116 and second 118 timers are illustrated as discrete devices, the timers can be integral to the data processing system 104. In addition, at least one of the first timer 116 and the second timer 118 could be implanted as a sample counter because the voltage and current sampling are typically performed synchronously by the data processing system 104 permitting the time intervals to be calculated from sample counts.
The phase determination apparatus and method used in the electric power meter provides an accurate and computationally uncomplicated system for determining the phase angle of a first waveform relative to a harmonically distorted reference waveform.
The detailed description, above, sets forth numerous specific details to provide a thorough understanding of the present invention. However, those skilled in the art will appreciate that the present invention may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuitry have not been described in detail to avoid obscuring the present invention.
All the references cited herein are incorporated by reference.
The terms and expressions that have been employed in the foregoing specification are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims that follow.
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|Cooperative Classification||G01R21/133, G01R25/08|
|Oct 20, 2004||AS||Assignment|
Owner name: VERIS INDUSTRIES, LLC, OREGON
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:BRUNO, DAVID A.;REEL/FRAME:015930/0594
Effective date: 20041005
|Jun 30, 2010||FPAY||Fee payment|
Year of fee payment: 4
|Nov 3, 2014||FPAY||Fee payment|
Year of fee payment: 8